1. Field of the Invention
This invention relates to a method and device for mounting an optical element to a shaft, whereby the optical element is rotated by the shaft. Specifically, this invention relates to a collet assembly for coupling a rotating optical element and a driving shaft together coaxially.
2. Description of the Prior Art
In many applications such as scientific instruments, it is necessary to mount an optical element such as a mirror, lens, prism, or diffraction grating on a shaft. The shaft may be driven by a small electric motor, so that operation of the motor rotates the optical element. The usual purpose is to reflect, diffract or refract a beam of light in a given direction. A typical application is shown in FIG. 1A, whereby incident light beam 80 from light source 82 strikes optical element 84 (which is a mirror here). Mirror 84 reflects the light beam 80 as shown to spot A. Mirror 84 is attached as shown to the end of shaft 86. Shaft 86 may be the shaft of small electric motor 88. Small electric motor 88 may be a galvanometer, which is a special kind of motor used for positioning purposes. Thus, instead of galvanometer 88 spinning continuously as might a conventional motor, electric current is provided to galvanometer 88 causing it to assume a desired position. This current is precisely controlled by instrumentation or a computer (not shown) so as to rotate shaft 86 typically just a few degrees, so as to reposition mirror 84 to a new position as shown in FIG. 1B. In FIG. 1B, light beam 80 is reflected to spot B.
In an optical system where an optical element 84 is to be rotated by some driving component such as motor 88, the element must be affixed to the shaft of the driver. Where the driving component is to be used to give precise positioning of the driven element it is important for the axis of the driving shaft of the driving component to be coaxial (i.e., in line) with the desired axis of rotation of the optical element. Further, if the optical element is to be positioned rapidly it is important for all components of the system to be low in rotational mass moment of inertial and to be balanced, so the axis of rotation passes through the center of gravity of the rotating element.
Known coupling techniques for shafts such as collars with set screws tend to shift the axis of rotation off center with the axis of the collar and may introduce imbalance due to non-symmetric construction. A pinch boss, another known technique, will typically add too much inertia to the system and may suffer from the same off center and imbalance problems as does a collar with a set screw. Thus, the need is to have a coupling technique that contributes little to the inertia of the system, is inherently balanced by being symmetric about the axis of rotation, and will not introduce any offset in the axis of rotation between the driving and driven components.